1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is directed to an improved connection between the lower nozzle of the fuel assemblies and the control rod guide thimbles.
2. Background Information
In most nuclear reactors the core portion is comprised of a large number of elongated fuel elements or rods grouped in and supported by structural frame works referred to as fuel assemblies. The fuel assemblies are generally elongated and receive support and alignment from upper and lower traversely extending core support plates. These upper and lower core support plates are directly or indirectly attached to a support barrel which surrounds the entire core and extends between the ends thereof. In the most common configuration, the axis of the core support barrel extends vertically and the various fuel assemblies are also arranged vertically resting on the lower support plate. Generally, in most reactors, a fluid coolant such as water, is directed upwardly through apertures in the lower core support plate and along the various fuel assemblies to receive the thermal energy therefrom. Conventional designs of these fuel assemblies include a plurality of fuel rods and control rod guide thimbles held in an organized array by grids spaced along the fuel assembly length and attached to the control rod guide thimbles. Top and bottom nozzles on opposite ends thereof are secured to the control rod guide thimbles; thereby forming an integral fuel assembly. The respective top and bottom nozzles extend slightly above and below the ends of the fuel rods, capturing the rods therebetween.
Generally, in each fuel assembly, there are a number of grids axially spaced along the fuel assembly length and traversely extending across the assembly. Convention designs of these grids include a plurality of interleaved straps of egg-crate configuration designed to form a plurality of cell openings, with each cell opening adapted to receive therethrough one of the fuel rods. A peripheral strap, being of the same height of the interleaved straps, encloses the interleaved straps to impart strength and rigidity to the grid. The purpose of these grids is twofold. One purpose is for the lateral support or positioning of the fuel rods so as to prevent localize neutron flux peaking and thereby permit operation of the reactor closer to its design power limit. The other purpose is for the mounting of deflecting vanes to promote mixing of the upwardly flowing coolant along the fuel rods to average the enthalpy/temperature rise for maximizing the power output of the reactor core. Normally, for lateral support or positioning of the fuel rods, each cell opening includes an arrangement of spring fingers and dimple protrusions which provide a six point contact of the fuel rods. For deflecting the coolant flow, some or each of the cell openings of the grids are provided with cantilevered deflecting vanes for deflecting the coolant. All of these prior art grids extend completely across the fuel assembly and separately surround each of the fuel rods contained in the assembly. Furthermore, the construction of each of these grids is such that its outer peripheral strap is of a height equal to a height of its inner straps.
The power output of a nuclear reactor is limited by the rate at which heat can be removed from the reactor core, and the rate of heat transferred determines the temperatures developed in a reactor core. Therefore, the maximum reactor operating power is limited by some enthalpy and/or temperature value in the reactor core. The variation of the neutron flux in the reactor core causes the fuel assemblies in the core to operate at different power levels, and this variation occurs even among the fuel rods within a single fuel assembly. The reactivity and, in-turn, the power output of a nuclear reactor is limited by the amount of structural material in the reactor core, as the structural material parasitically absorbs neutrons which could otherwise be used in the fission process. Furthermore, a reduction of the structural material in the fuel assembly reduces the pressure drop and thereby increases the power output. Still further, it is well known, that the burn up rate for the different fuel rods contained in a given fuel assembly varies. And still further, the output of the given fuel assembly can be enhanced by the use of different fissionable materials, as well as, by the amount of fissionable material, such as through the use of different diameter sized fuel rods. With these considerations in mind, designers are constantly striving to improve upon the power output of the various fuel assemblies which make up a core to increase the total power output of the reactor, while at the same time, striving to improve on the construction of the assemblies so as to facilitate the assembly of the fuel assembly and to reduce the repair and maintenance cost associated with operating the reactor. Some of these costs are associated with the fixed components of the fuel assembly forming the fuel assembly skeleton. The skeleton comprises essentially the upper and lower nozzles with the guide thimbles and instrumentation tube rigidly connected therebetween and the spaced, tandem arrangement of grids fixedly connected to the guide thimbles between the upper and lower nozzles. Currently, in one design of a fuel assembly, the control rod guide thimbles are secured to the lower nozzle through a screw that extends through the lower nozzle into the guide thimble. The guide thimble screw is retained in position by a circular locking disc that mates with a slot in the thimble screw head and is welded to the inside of the bottom nozzle leg counterbore through which the screw extends. The welding procedure requires the ability of a skilled operator and inspection of the finished product is very difficult because of the deep recess in the bottom nozzle leg counterbore. This weld procedure is performed after the fuel assembly is loaded with fuel rods, thus raising the level of difficulty in completing the task as well as increasing the difficulty of recovery should an improper weld be performed.
Accordingly, an improved attachment mechanism is desired to secure the control rod guide thimble to the bottom nozzle that does not require welding.
Furthermore, a new attachment mechanism is desired that will fasten the control rod guide thimbles to the bottom nozzle that can easily be re-worked if a defective connection is identified.
Additionally, a new connection between the bottom nozzle and control rod guide thimbles is desired that requires fewer components and can be more easily inspected.